code: def unchain_backward(state):
      """Call Variable.unchain_backward recursively."""
      if isinstance(state, collections.Iterable):
          for s in state:
              unchain_backward(s)
      elif isinstance(state, chainer.Variable):
          state.unchain_backward()
  
imports: from abc import ABCMeta
  from abc import abstractmethod
  import collections
  import contextlib
  import chainer
label_desc: Use image node_id 1 for calling a global function with example usage: unchain_backward(state) without return types